Make canvas overflow hidden.

Even though the sizer is scaled, it's original 'ghost'
size is used to hint scrollbars in Firefox (unlike in Chrome).
Therefore the canvas's overflow needs to be hidden to
prevent this ghost size leak.
Re-apply inline-block to canvas element on resizing.
Chrome tends to forget this property after a relayout.
Do not show a scrollbar in slideshows in the viewer.
Slideshows are (currently) meant to be navigated by
the nav buttons.
Let sizer overflow out of the canvas during
a pinch-zoom gesture.
Center all slideshow pages
Move slideshow CSS customizations for canvasContainer to viewer.css
Apply presentation mode styling via classList.add,
and not by setting the className.
Merge pull request #589 from adityab/scrollfix

Polish and papercut-fixes for the viewer